Dallas was influential as the presiding officer of the Senate, where he worked to support Polk's agenda and cast several tie-breaking votes. He called for the annexation of all of the Oregon Territory and all of Mexico during the Mexican–American War, but was satisfied with compromises that saw the United States annex parts of both areas. Though Dallas was unsuccessful in preventing Polk from appointing Buchanan as Secretary of State, he helped convince Polk to appoint Robert J. Walker as Secretary of the Treasury. As vice president, Dallas sought to maneuver himself into contention for the presidency in the 1848 election, as Polk had promised to serve only one term. However, Dallas' reluctant vote to lower a tariff destroyed much of his base in Pennsylvania, and his advocacy of popular sovereignty on the question of slavery strengthened opposition against him.
daLlaS was InfLUentIaL as tHE prESiding oFFiCEr of tHe SenatE, WhEre he wORKEd tO supPorT PolK's ageNda aNd cAsT sevErAl Tie-bREakIng voteS. He callEd fOr tHe ANnEXaTIOn oF aLl oF ThE orEGon terrItoRY anD aLL of meXico DurINg The MexicaN–amErican war, But Was sATiSfIed WITH ComPrOmiSeS that saw thE unitEd stATes ANnEX PaRts of BOTh arEAs. thOUgh DALlas waS UnsucCeSsful in PrEveNting polk From apPoiNting buchanAn as secretaRy oF State, he hElped conviNce Polk to Appoint robErt j. wAlkEr as secrEtAry Of the treasUry. as vice preSidENt, DallaS SOught to maNEuvER Himself INto COnTention For the presidency iN tHE 1848 elecTion, as PolK had promised to sERve Only one teRm. HowEver, DAlLas' RELUCTanT voTe to LOwer A tariff deStRoyed Much of his base in peNnsYlvANia, ANd hIs aDvoCacy oF popUlaR sovereigNty On The qUestiOn of Slavery strengTheNEd opPOsitIoN against him.