The slate of competitive congressional primaries could showcase the strength of party establishment and the power of a presidential endorsement.

WASHINGTON — A series of high-profile Republican retirements and a growing base of Democratic voters have made congressional races in Texas some of the country’s most crowded and closely watched.

Primary races that will be decided on Tuesday are expected to move a new generation of Republicans closer to taking office and to narrow the field of Democratic candidates vying to challenge conservative incumbents.

Here’s what to watch for in the Texas congressional primaries.

Will the party establishments be able to protect their most vulnerable incumbents?

Two of the most powerful incumbents in the Texas delegation are facing challenges from opponents who call them too moderate for their parties, even as the Washington establishments fight to keep them in Congress.

Representative Kay Granger, the Republican conference’s most senior woman, is facing a challenge from Chris Putnam, a former technology executive. Mr. Putnam has targeted Ms. Granger for critical comments she made against Donald J. Trump during the 2016 election and for her role negotiating billions of dollars in government spending as the top Republican on the House Appropriations Committee. President Trump has endorsed her, but Ms. Granger is in the fight of her political life.

Representative Henry Cuellar, Democrat of Texas and also a member of the Appropriations Committee, is trying to fend off a challenge from Jessica Cisneros, an immigration lawyer. Ms. Cisneros was handpicked and championed by Justice Democrats, the insurgent progressive group that recruited and backed Alexandria Ocasio-Cortez, now a New York representative, who has also endorsed her.

The race, which has been flooded with outside money, will serve as an early test of the nascent left flank’s power as it tries to oust Mr. Cuellar, one of only three House Democrats with an “A” rating from the National Rifle Association.

Leading Democrats have scrambled in recent days to shore up support for Mr. Cuellar, with Speaker Nancy Pelosi making a visit to his district.

“My role is to win the House for the Democrats and to do so by winning for our incumbents,” Ms. Pelosi said on Friday. “I would like them to get a nice win so that there’s a message for the future, for the next election and the rest.”

The message, she added, “would be that members who come here, who help us win the majority, will have the support of the leadership of the House.”

How powerful will endorsements — including the president’s — be?

Mr. Trump, both on Twitter and at his “Keep America Great” rallies, has doled out endorsements to Texas incumbents and his chosen candidates in the races. Aside from Ms. Granger, he has given a nod to his former White House physician, Dr. Ronny Jackson, who is seeking a seat in the state, and to August Pfluger, a former National Security Council adviser who is running to represent the 11th District.

But in Ms. Granger’s race, which has also been flooded with outside money, her opponent has sought to play down the endorsement as a ploy on the part of the president’s to ensure that there were no defections during the impeachment vote.

A number of Republican lawmakers in the state who are retiring have also tapped successors: Representative Will Hurd, for example, has singled out Tony Gonzales, a Navy veteran, to take his seat representing the 23rd District, and Representative Bill Flores has selected Renee Swann for the 17th District.

Who will mount a successful comeback?

A number of Texas races feature candidates who are seeking to return to Congress or to have another shot at ousting an incumbent.

Pete Sessions, a Republican who lost his seat representing the 32nd District in 2018, is now running for an open seat in the much more conservative 17th District. The race is expected to go to a runoff.

In 2018, Sri Kulkarni, a former diplomat and a Democrat, came within five percentage points of ousting Representative Pete Olson from his seat in the 22nd District, which covers the southern suburbs of Houston. With Mr. Olson’s retirement, Mr. Kulkarni is back in the race — as is Gina Ortiz Jones, who lost in 2018 to Mr. Hurd by less than 2,000 votes.

M.J. Hegar, an Air Force veteran who unsuccessfully ran for a House seat in 2018, is now seeking to secure the Democratic nomination to challenge Senator John Cornyn, a Republican, in the state’s Senate race. Ms. Ocasio-Cortez has endorsed Cristina Tzintzún Ramirez, a labor and voting rights activist who is more progressive, but Ms. Hegar has the support of Democrats’ Senate campaign arm.