We have an opportunity to move away from politics as usual and toward fresh ideas and new leadership, Roberts said in his official announcement Monday.
Zelenik and Roberts may have to fight off more than one member of the Tennessee state Senate in the GOP primary.
State Sen. Diane Black (R) told state newspapers Monday that shes also considering the contest.
Other Republicans already in the race include retired Army Reserve Maj. Gen. Dave Evans and Realtor Gary Mann.
While national Republicans had expressed an early interest in Zelenik, insiders said Monday that Gordons exit from the race makes Washington involvement in a primary less likely.
That may be a sign of just how confident the party is about the 6th district since the National Republican Congressional Committee has given every indication it intends to stay involved in the 8th district GOP primary as it looks to target the seat of Rep. John Tanner (D-Tenn.), who announced two weeks ago that he would retire in 2010.
Talley, the Tennessee Democratic Party spokesman, expressed his hope that Republicans will get caught up in a nasty primary and said the state party will work to keep any Democratic primary from becoming a bruising affair.
Democratic Congressional Campaign Committee Chairman Chris Van Hollen (Md.) said the party can retain the seat with a Democrat who shares Chairman Gordons commitment to putting progress before partisanship on behalf of Middle Tennessee.
Among the early names being floated by Democratic insiders as possible replacements for Gordon in the 6th district are state Rep. Mike McDonald, Wilson County Sheriff Terry Ashe, state Rep. Hank Fincher, Murfreesboro Mayor Tommy Bragg and former state Sens. Andy Womack and Jo Ann Graves.
McDonald acknowledged Monday afternoon that he was mulling jumping into the race.
I have had a number of phone calls from Democrats here in the state asking me if Id be interested in running and some cases encouraging me to run, he said.
Fincher said he was definitely considering the race and added that just because the district is conservative doesnt mean that Republicans will have an advantage in the open-seat contest.
Republicans dont have the exclusive claim to conservative values in Tennessee, Fincher said. I think a Democrat who has the independent, conservative values of this district will go toe-to-toe with anyone the GOP puts up.
United We Dream protesters carry a mock coffin to the office of Sen. Ted Cruz, R-Texas, in the Dirksen Senate Office Building on Monday, July 21, 2014, to hold one of their "funeral services for the Republican Party" due to GOP positions on immigration. The immigration reform group visited several other Senate Republican offices to hold similar funeral services.