The White House takes its onslaught on vulnerable Senate Democrats to two Midwestern states Thursday night as Republicans try to expand their majority in the chamber.

President Donald Trump will hold a rally in Indiana, where incumbent Democratic Sen. Joe Donnelly faces a challenge from GOP businessman Mike Braun in a state the president carried by about 20 percentage points in 2016. Vice President Mike Pence will campaign in Wisconsin for Republican state Sen. Leah Vukmir, who aims to take out Democratic Sen. Tammy Baldwin in a state the president won by a razor-thin margin.

Pence's trip to the Badger State follows a Wednesday fundraising stop in Michigan for Republican veteran and businessman John James. He is challenging Sen. Debbie Stabenow, a Democratic incumbent who faces re-election in another state that Trump only narrowly won.

The White House's Midwestern blitz underscores the wealth of opportunities the GOP has to try to flip Democratic Senate seats this year. Democrats and independents who caucus with them have to defend 26 seats, including 10 in states the president won in 2016, giving them a slim chance of taking control of the Senate. Trump has repeatedly complained that Republicans' current 51-49 seat majority in the Senate is too narrow for him to accomplish his policy goals, such as repealing the Affordable Care Act.

Trump's and Pence's visits also highlight the varying strategies Democratic Senate incumbents have had to adopt as they fight for their political survival in distinct states. While most Senate Democrats defending their seats in Trump states have prioritized health-care policy, some in the darker red states have also tried to tie themselves to Trump both through their words and votes.

Take Donnelly, who is considered one of the most endangered senators in his party this year. He has voted with Trump's priorities more than half the time, and repeatedly emphasized his efforts to work with Republicans. A Donnelly campaign ad released this week highlights the senator's vote for Trump's immigration package this year "to build a wall and protect our borders," over video of him shaking the president's hand.

Trump has hardly returned the flattery. When he visited Indiana in May, he dubbed Donnelly "Sleepin' Joe" and contended that he is too liberal for Indiana. Though Donnelly has supported Trump on immigration, judicial nominations and other issues, the president has focused his ire on unified Democratic opposition to both last year's failed efforts to repeal the Affordable Care Act and the GOP's tax overhaul that passed in December.

While independent polling has been limited so far, most signs point to a close race in Indiana.