State Speaker Chris Donovan is expected to launch a Congressional bid today in Connecticut’s 5th district.

The Democrat becomes the instant favorite to replace Rep. Christopher Murphy (D), who is running in the open-seat Senate contest. Donovan joins a field that already includes former state Rep. Elizabeth Esty.

Donovan will formally announce his candidacy from the steps of Meriden City Hall, Roll Call has learned.

Republicans see an opportunity in the 5th, which is typically one of the state’s more competitive districts. But the GOP has a primary on its hands as well.

Lisa Wilson-Foley, a former candidate for lieutenant governor, is running for the GOP nod, as are former 5th district candidates Justin Bernier and Mark Greenberg. Sam Caligiuri, the 2010 nominee, won’t run again.