The most expensive Senate primary in history will finally come to a close on Tuesday when Texas Republicans choose between incumbent Sen. John Conryn and state Attorney General Ken Paxton in the runoff.Several other races from the March 3 primary are also on the runoff ballot Tuesday, including attorney general, state railroad commissioner and a number of congressional races. Here are some of the major races to watch: Texas Senate GOPCornyn and Paxton are facing off in the primary runoff after neither received a majority in the March primary. The race could have a seismic impact on the Senate — especially after President Trump last week threw his endorsement behind Paxton. Cornyn, first elected in 2002, came out as the top vote-getter in the three-way primary in March but failed to secure at least 50% of the vote. The former Republican whip and chair of the Senate GOP fundraising arm, Cornyn had sometimes been at odds with Mr. Trump, although he has been an ally to the president in his second term. Paxton, meanwhile, had been a staunch supporter of Mr. Trump for years.No Democrat has won statewide office in Texas since 1994, making whoever wins Tuesday's runoff the favorite to win in November. Mr. Trump's lack of endorsement had been a defining feature of the race, with reports floating after the primary that he would back Cornyn. Two days after the primary, Mr. Trump said he would endorse "SOON" and called on whoever he chose not to endorse to drop out. Paxton, meanwhile, said he would only drop out after the Senate passed a voter ID law backed by Mr. Trump known as the SAVE America Act.