That depends on which Administration happened to be in Government when the "Kingfish" made his move - if it were President Hoover then I think we can safely say "No" given the latter's disinclination to start Wars and I'd be surprised if the Smith Administration were to make such an aggressive move without first suffering Serious Provocation from Jake Featherston. Having said that, if Governor Long DID declare the secession of Louisiana from the CSA then elements within the United States of America (especially the Armed Forces & Democratic Party) would certainly make some unofficial effort to support the Louisiana Republic against the Confederacy.
The only question is whether that effort would be enough to sustain Long against The Snake for long enough to awaken some Moral Outrage across the North or (better yet) trigger a Diplomatic Incident serious enough to pull the United States into a fourth War against the South; it is also interesting to wonder if this Louisiana Secession would be enough to delay or outright deny Featherston's chance to bring his curses Home against the United States on HIS terms (given that even a relatively small-scale Civil War is likely to damage his prestige and Southern Infrastructure).
In fact this is actually quite a fascinating question and might be the seed of an Alternate Timeline in its own right, so congratulations to you for raising it CCA!
The only question is whether that effort would be enough to sustain Long against The Snake for long enough to awaken some Moral Outrage across the North or (better yet) trigger a Diplomatic Incident serious enough to pull the United States into a fourth War against the South; it is also interesting to wonder if this Louisiana Secession would be enough to delay or outright deny Featherston's chance to bring his curses Home against the United States on HIS terms (given that even a relatively small-scale Civil War is likely to damage his prestige and Southern Infrastructure).
In fact this is actually quite a fascinating question and might be the seed of an Alternate Timeline in its own right, so congratulations to you for raising it CCA!