President Trump is scheduled to attend a luncheon in Houston next month for the National Republican Senatorial Committee. (Evan Vucci/AP)

President Trump is slated to headline a fundraiser to benefit Senate Republicans in Texas on May 14, according to an invitation obtained by The Washington Post, as attention in the party shifts more urgently toward preserving control of the upper chamber of Congress in the midterm elections.

The president is scheduled to attend a luncheon in Houston for the National Republican Senatorial Committee, which is the Senate GOP’s campaign arm, according to the invitation. Contribution levels range from $5,000 per person to $100,000 per couple, the invitation says.

Representatives for the NRSC and the White House did not immediately comment. Details of the event were confirmed with The Post by two people with knowledge of it. They confirmed it on the condition of anonymity because it had not been announced publicly.

Republicans are defending a 51-to-49 Senate majority in the November elections. There is growing worry in the party that Democrats will win control of the House. Increasingly, Republicans view the Senate as a potential last line of defense against Democrats on Capitol Hill beginning in 2019.

Speaker Paul D. Ryan (R-Wis.) announced Wednesday that he will retire at the end of his term, a move seen by some Republicans as the latest sign of danger for party as it tries to keep its power in the House.

Still, Trump is also slated to raise money later this month for the National Republican Congressional Committee at his Mar-a-Lago Club in Florida, people familiar with the plans said. The NRCC is the House Republican campaign arm.

But party leaders have greater confidence in the Senate map than they do in the House race landscape. Democrats are defending far more Senate seats than Republicans this year. Many are in ruby-red states Trump won in 2016.