Kris Kobach Is Back, and a Kansas Senate Seat May Be Up for Grabs

“There are lots of people that have been concerned about this,” said former Gov. Jeff Colyer, a Republican whom Mr. Kobach narrowly defeated in the 2018 gubernatorial primary. “I’m supporting Congressman Marshall because he’s the one who can most readily, stably defend the seat.”

Mr. Marshall, an obstetrician-gynecologist, is emphasizing his strong opposition to abortion rights, his work on agricultural matters and his support of Mr. Trump.

“If we Kansans send the wrong person to the general election, it could be competitive,” Mr. Marshall said in an interview. “I just don’t think Kansans will make that mistake twice.”

Asked about engaging moderate Republicans who are uncomfortable with Mr. Trump, he replied, “Well, goodness, that wouldn’t be very many.” But, he said, “you compare and contrast. Do you want the left’s radical socialist agenda, or President Trump’s great economy and national security?”

A recent poll conducted by the firm Public Opinion Strategies — for Mr. Marshall’s campaign — showed him with a primary lead over Mr. Kobach, a reversal of March numbers that had shown Mr. Kobach, who is well known in the state and boasts a base of devoted supporters, ahead.

But some national Republicans are unenthusiastic about Mr. Marshall, and underwhelmed by his fund-raising. The political arm of Club for Growth, a conservative outside group, is planning an ad campaign of around $2 million that will cast Mr. Marshall as aligned with the “Mitt Romney wing of the party,” and will criticize his record on matters like big spending, said David McIntosh, the group’s president. Mr. Marshall has countered by saying he votes with Mr. Trump.

The group has not yet endorsed Mr. Kobach, whom Mr. McIntosh called a “strong conservative,” saying earlier this month that the organization was reserving judgment to see what kind of campaign Mr. Kobach would assemble.