After the failure of The Slidell Mission in May 1846, Polk used information about skirmishes inside the disputed territory between Mexican troops and Taylor`s army to gain congressional support for a declaration of war against Mexico. On May 13, 1846, the United States declared war on Mexico. During the debates of the House of Representatives, constitutional objections were expressed as to whether both houses of Congress could constitutionally authorize the admission of territories and not states. In addition, the Republic of Texas, a nation in its own right, as a state, its territorial boundaries, property (including slave ownership), debt and public lands, would require a treaty ratified by the Senate.  Democrats were particularly concerned about taxing $10 million of Texas debt on the United States and were upset by the flood of speculators who had bought cheap Texan bonds and now used Congress for the Texas House bill.  Democrats in the House of Representatives gave the legislative initiative a dead end to the Southern Whigs.  After the success of Texas` war of independence against Mexico in 1836, President Martin van Buren renounced the annexation of Texas after the Mexicans threatened to wage war. Although the United States recognized diplomatic recognition in Texas, it did not take further annexation measures until 1844, when President John Tyler resumed negotiations with the Republic of Texas. His efforts culminated on April 12 with an annexed treaty that led Mexico to abolish diplomatic relations with the United States. Tyler, however, did not have the votes in the Senate to ratify the treaty, and he was defeated by a long time in June.
Shortly before leaving office, Tyler tried again, this time by a joint resolution of both houses of Congress. With the support of President-elect Polk, Tyler succeeded in passing the joint resolution on March 1, 1845, and Texas was admitted to the United States on December 29. Members of the Anti-Texas Whig had lost more than the White House in the 1844 legislative elections. In the southern states of Tennessee and Georgia, the Whig strongholds in the 1840 general election, the electorate`s support for pro-annexation in the Deep South quickly declined – and Clay lost every Deep South about Polk.  The intransigent hostility of the Northern Whigs to the expansion of slavery increasingly characterized the party, and the members of the South had suffered by the union of the accusation of being «soft in Texas, thus soft in slavery» of the Southern Democrats.  In light of the 1845 congressional and governor elections in their home countries, some Southern Whigs attempted to suppress this impression of the Tyler Texas Act.   After the capture of Mexico City in September 1847, Nicholas Trist, chief secretary of the Ministry of Foreign Affairs and Polk`s peace envoy, began negotiations for a peace agreement with the Mexican government under conditions similar to those of the previous year`s Slidell.