Thirty-four seats in the U.S. Senate are up for election on Tuesday, putting the balance of power between Democrats and Republicans in question.

Republican Sen. Ted Cruz, of Texas, sought to fend off an underdog challenge Tuesday from Democratic Rep. Colin Allred in one of the year's most expensive races, which is testing shifts in America's biggest red state and could factor into the fight for U.

Texas voters could decide control of the United States Senate this year. Democratic Congressman Colin Allred is challenging incumbent Republican Sen. Ted Cruz for Senate in a race that has attracted national attention.