John Decker, Republican challenger for the 1st Congressional district, will be holding a campaign event in Hartford tomorrow, during which he will "officially announce his candidacy:
This from Decker's Facebook page:

Please join us tomorrow, March 28th, from 12-1 at CityPlace in Hartford (185 Asylum St) for John's official live public announcement of his candidacy for Connecticut's 1st Congressional District.

 Decker's candidacy was previously announced by Dennis House of Channel 3, and he's not the only GOP candidate trying to unseat John Larson. Mike McDonald is also vying for the 1st district seat. 
In 2010, Larson defeated Republican Ann Brickley (who had beat out Mark Zydanowicz during the primary), with 61.25 percent of the vote to Brickley's 37.20 percent.