DENVER — They keep expecting to see Senator Cory Gardner everywhere — on the local Fox affiliates in Colorado, on Facebook, on literature crammed inside their mailboxes. They are voters who wear tasteful crepe blouses and carry structured Kate Spade totes, who like how their 401(k)’s are performing but say they could do without President Trump’s “temperament.”

They are members of one of the most coveted groups in electoral politics: suburban women. But in their field of vision, Mr. Gardner, Colorado’s top Republican officeholder, is almost nowhere to be found.

“I don’t hear him speaking out on things,” said Jennifer Gremmert, 50, the executive director of an energy nonprofit. She is the kind of voter who could help Mr. Gardner win re-election in November, a registered Democrat who considers herself “nonpartisan,” “not that enthusiastic” about her party’s Senate candidates, and “totally” open to Mr. Gardner. But when it comes to the bipartisan stands that Ms. Gremmert said she prized in a candidate, “I don’t see him.”

On one level, this is strange: Many of these voters were crucial to Mr. Gardner’s narrow Senate victory in 2014, when he carried the suburban vote and was ahead among independents, according to exit polls. And they may be even more essential to him now — he is widely considered to be one of the most at-risk G.O.P. senators seeking re-election this year.