John A. Boehner, the House speaker, plans to visit upstate New York on Monday to support the struggling campaign of a Republican candidate who is running for a vacant seat in Congress.
Mr. Boehner’s appearance underscores the national stake in the race, and reflects the changing fortunes of the candidate, Jane L. Corwin, a popular state lawmaker who experts in both parties had once thought would win handily in the 26th Congressional District, which is heavily Republican.
In recent weeks, Ms. Corwin has watched her advantage in the race all but disappear as her Democratic rival, Kathy Hochul, seized upon public uneasiness over the House Republicans’ plan to overhaul Medicare, the federal health insurance program for retirees.
Ms. Hochul, the Erie County clerk, has repeatedly attacked the plan, introduced last month, as a threat to the elderly and has called on Ms. Corwin to reject it. Ms. Corwin has remained steadfast in her support of the Medicare plan.
Ms. Corwin’s position has placated conservatives, who stand to play an important role in the election, as both campaigns rely on their political bases to generate turnout in a special election that may not register with the larger electorate.
The personal interest shown in the race by Mr. Boehner — who will host a fund-raiser for Ms. Corwin and also make a public appearance with her — reflects the growing concern among Republicans about a race that they had not expected to be so competitive.
The visit also comes as national Democrats, who had all but written off the race a few weeks ago, now view an opportunity to turn it into a referendum on the House Republican agenda in advance of the battle for control of the House in the election next year.
With the Medicare debate clearly providing Ms. Hochul momentum, her campaign has signaled it has no intention to drop it. While Mr. Boehner meets with Ms. Corwin at Salvatore’s Italian Gardens restaurant in Depew, Ms. Hochul plans to hold a news conference nearby with the National Committee to Preserve Social Security and Medicare.
The district includes the northeastern suburbs of Buffalo and the western suburbs of Rochester, as well as Albion, Batavia, Lockport and North Tonawanda. It has about 30,000 more registered Republicans than Democrats and is considered one of the state’s more conservative regions.
The election, on May 24, is to fill the seat of Christopher Lee, a Republican who resigned in February after he e-mailed a woman a shirtless photo of himself that appeared on the Web.