McCain/Lieberman 2000 has that ring of consensus landslide -- think Obama '08 or Bush '88 -- to it, doesn't it? It was my first take, too. Consider, though, three factors in replacing Bush 2000 with McCain:
1. You're replacing one of the very best campaign strategists of the modern era (Karl Rove) with some combination of Mike Murphy and Rick Davis, who aren't;
2. You're potentially butterflying away the Ralph Nader candidacy, which forced Gore to spend money in liberal strongholds like Oregon (!) and generally impacted the view of Gore throughout the general election; and
3. You're potentially strengthening a third-party conservative run.
If you want to argue that Nader and Buchanan run anyway, then the question boils down to whether the superior candidate skills of McCain 2000 outweigh the superior infrastructure and strategic skills of Bush 2000. Here's just one decision to think about: Rove spent serious money campaigning in Tennessee (11 EV), New Hampshire (4 EV), and, most insanely, West Virginia (5 EV), despite the facts that those states were (a) Al Gore's home state, (b) a state Clinton-Gore won by 10 points in the last election cycle, and (c) a state Clinton-Gore won by 15 points in the last election cycle and a long-standing Democratic stronghold that voted for Dukakis in '88.
No other Republican candidate would have spent time and money in those three states in 2000, IMO. IOTL, Bush 2000 won NH by 1%, TN by 4%, and WV by 6%; without Rove calling the shots, I think all three of those go uncontested by the Republicans. On the flip side, Gore isn't going to contest neighboring New Mexico (a narrow OTL victory) and its 5 EV, and Lieberman probably helps McCain win Florida outright. That puts the Gore/Kerry base as 282 electoral votes ITTL.
The next closest Gore 2000 states are Iowa, Wisconsin, and Oregon; the latter two were close only because of Ralph Nader eating into Gore's left flank. If you give McCain-Lieberman Iowa, that brings Gore/Kerry down to 275 EV.
After that, the next plausible battleground is Pennsylvania, which gave OTL's Gore + Nader a collective 52.7% of the vote, or a 6.5% lead over Bush. Could that be the basis for an alt-2000 analogue, where teams of lawyers rush to Harrisburg to count hanging chads? Would moderate Republican Governor Tom Ridge order a statewide recount?