If you had been a member of Congress in 1846, how would you have voted on the question of war with Mexico?
I would have voted to go to war as well. Texas was part of the United States, and despite the discrepancies in the border between the two nations, the people believed that they were protecting their territory.
Since Texas had revolted in 1836 and the U.S. had annexed Texas in 1845, when Congress voted in 1846, yes, I would have voted to battle against Mexico. Mexico had already lost Texas. Why give it back even if Mexico considered it yet as part of their territories? (Of course, this is but one issue, but it would seem weakness to return Texas to Mexico.)
I think if I were a member of Congress at that time I would have voted to go to war with Mexico. I think that Manifest Destiny would have been a belief of anyone in Congress at that time. It was also a time when the US was wanting to expand its territory.
I have changed your question from asking about a war with France because, as far as I know, there was no question of the US going to war with France in 1846.
If I had been in Congress in 1846, I imagine that I probably would have voted for war with Mexico. I would have done so because in those days, people really thought it was okay to take territory from others by force. This was especially true with us and Mexico because the idea of Manifest Destiny made us think that we really deserved to have all of that area that was then part of Mexico. I'm sure I would have thought like the average American on that issue.
If I had been in the North, I might have worried some about slavery spreading. But I think that the idea of getting more territory and of showing our power as a country would have made me vote for the war.