And by against him, I mean pretty much anyone could have run against him and win. In your scenario, I'd say Carrio probably wins - Lopez Murphy is too right wing and Rodriguez Saa might not postulate.
However, let's take a go at this with an earlier pod.
By late 1993, Menem tries to reach an agreement with the opposing UCR regarding a major constitutional reform which, among other things, would include the possibility of immediate reelection for the president. After long discussions and at the brink of breaking apart, the UCR chooses to support a constitutional reform but, ITTL, as long as it doesn't include reelection. The bill is passed in the Congress and elections are called to vote for the representatives who would reform the constitution. Despite the agreement between the ruling PJ and the opposing UCR, more hotheaded sectors of the PJ state they would try to enable reelection anyway.
However, as reelection was a somewhat unpopular topic and with the election polarized about it, the UCR wins the election in most districts. The Constitutional Convention takes several months to vote modifications. At the end, the presidential term is shortened to four years, but immediate reelection is not passed. In return, the PJ and its allies vote against having a senator for the minority, meaning only the winning party would get to put senators in the Congress, instead of the 2 for the winning party and one for the second place as the UCR intended.
By 1995, Menem is thus prohibited from running for presidency and the governor of the province of Buenos Aires, Eduardo Duhalde, is elected president, with the economy suffering the effects of the Tequila effect. By early 1996, unemployment has risen to 20% and poverty is rising to levels never seen before. As the economy begins to recover, the Asian Tiger crisis hit, as well as the Russian crisis. By the late '90s, in a date the author is too lazy to research, Brazil devaluates the Real.
With the economy about to enter a recession, president Duhalde tries to start a controlled devaluation of the Argentine Peso. This measures begin to trigger inflation and, given the uncertainty of the coming 1999 presidential elections, the Peso devaluates fast, loosing half it value in a month. Demonstrations against the economic measures quickly ensue. This author, having been screwed out of his graduation trip thanks to the devaluation, joins in them, along with pretty much every 17-18 years old boy and girl in the country. As he's writing this story, his course gets to participate in a tv show to win a free trip and he, of course, saves the day, but that's another story.
Back to politics, the UCR wins the 1999 presidential elections in a landslide, and Fernando de la Rua becomes president in December 1999. What follows, though, it's not good. The devaluation of the Peso has given competitiveness to the Argentinean economy, at the cost of the worker's purchasing power. As inflation rise and employment doesn't initially recover, poverty rises. Foreign debt, both public and private, being in dollars has risen as well. De La Rua negotiates over and over with the IMF, which refinances the debt but, in return, demands budget cuts which cause huge political upheaval and hurt the economy.
Peronist unions jump at the opportunity and stage strike after strike. With food prices on the rise, the media quickly points out at the near starving children across the country. There are more beggars than ever, plenty of poor people begin recollecting trash for recycling and a few bucks. Public opinion and, specially the peronist party, demand welfare programs to help the poor. Trying to maintain his agreements with the IMF, the government programs are too few and too little. Carlos Ruckauf, governor of the province of Buenos Aires, issue his own welfare program. The eligibility for this program is pretty much determined by peronist militants working in the slums and there is no clear rule about how the benefit can be removed. As such, the program is nothing short of buying votes in exchange of bread. Rumors of a tuberculosis epidemic are also spreading. Corruption scandals shake the government every two months. The media talks about "touristic strikes" - not a single holiday passes without a strike the day after or before.
The Peronist party begins their internal struggles to determine the president for the 2003 elections - the options seem to be reduced to former presidents Menem or Duhalde. A few deaths, including yet another journalist, are thought to be linked to these PJ's internal struggles. The repeated general strikes are believed to have eaten as much as 2% of GDP growth. Unemployeds, of course unable to make strike, go blocking major avenues and bridges as they claim for state aid. Everyone believes the peronist governors are supporting and enticing them to do so. Not a week passes without a major access to the city of Buenos Aires blocked. In the rest of the country, repression is the norm and people killed in demonstrations is turning normal.
Overall, De La Rua is seen as a soft and mostly incapable president. Society demands a strong leader, who would not starve children to appease the IMF, who can control inflation and who would put Hugo Moyano and other union leaders in their place (aka as "jail"). Instead, he tries to appease everyone, and fails. By the 2001 mid term elections, he has no support from anyone, not even his own party.
Despite the economy growing at 3% and inflation lowered to 15%, the PJ wins the elections, with the UCR making a disastrous election in most districts. Several politicians, led by Elisa Carrio, splinter away from the sinking party. With both houses of the Congress with a peronist majority, no major laws can be passed and De La Rua would be hand binded, should he cared to use his hands to begin with, of course. Eduardo Duhalde seems to be about to become the peronist candidate for the 2003 elections as he secures the support of the governors of Cordoba, Santa Fe and Buenos Aires. And then, he screws up.
Looking to regain some sort of popularity, by 2002 De La Rua had sent a bill to the Congress to veto the amnesty laws of the 80s, so the criminals of the last dictatorship can be put on trial. Given the sensitivity of the subject, both houses unanimously pass the bill, and former torturers and leaders of the dictatorship begin to be arrested across the country. Duhalde, feeling confident, decides a bit of controlled military upheaval can help further destabilize the government and negate what looks like the single good thing De La Rua managed to do in office. However, as he's finishing a deal with General Brinzoni, he's suddenly arrested.
Brizoni, when first contacted, wanted nothing to do with such a plot, as he was trying to let the past behind. He fools Duhalde into believing he'll let or even command some sort of small, non consequence, rebellion. But actually, he make an agreement with Menem to keep his job as head of the army and joint staff if he discredit Duhalde. As he's caught in the act, no impeachment is required for senator Duhalde. Menem pays (with public money, of course) the judge to make sure Duhalde is given preemptive prison and has to wait his trial in jail.
Come 2003, Menem wins the presidential elections under promises of stability and a firm hand.
That ended up long
I know that Menem would be defeated by Nestor Kirchner in a hypothetical run-off.
My question, had Nestor Kirchner never runs, who do you think would be Duhalde's candidate for the presidency?
Well, whoever wanted to get that hot potato. Das Neves perhaps? All the important peronist leaders turned down the offer.