Their campaign was brief - Andrews announced April 2 - but pointed, featuring television ads from both and two debates.
Andrews, 50, who has served in the House since 1990, reminded voters that Lautenberg would be nearly 91 at the end of his next term if re-elected. Meanwhile, Lautenberg highlighted that Andrews was among the authors of the bill authorizing the Iraq war.
Lautenberg and Andrews have largely ignored a third Democrat seeking the nomination, Morristown Mayor Donald Cresitello.
The winner will compete in November's general election against one of three Republicans: state Sen. Joseph Pennacchio, Murray Sabrin, a business professor at Ramapo College, or former Congressman Dick Zimmer.
Political experts say Lautenberg and Zimmer - who each have support from much of their party regulars - are likely to prevail, with the incumbent favored for re-election in Democratic-leaning New Jersey.
The state has not elected a Republican to the Senate in 36 years. Democrats outnumber Republicans by a 5-to-3 margin, although unaffiliated voters account for nearly half of those registered.
Voter turnout is expected to be low, especially since this year the state's presidential primary was held in February instead of June.
Polls are open 6 a.m. to 8 p.m. Tuesday. Registered Democrats and Republicans can vote in their party races. People who are registered but unaffiliated must choose a party at the polls if they wish to vote.
The winners advance to the Nov. 4 general election, when all 13 of New Jersey's House seats and one of its two U.S. Senate seats will be decided.
Only two of New Jersey's House incumbents face primary challenges: Rep. Frank LoBiondo, a Republican in south Jersey's 2nd District, and Rep. Rodney Frelinghuysen, a Republican in north Jersey's 11th District.
The most closely watched House primaries are those involving Republicans in the 3rd and 7th districts, where incumbent GOP congressmen are stepping down.
Eight Republicans - including state Sen. Leonard Lance and Kate Whitman, daughter of former Gov. Christie Whitman - are competing for the 7th District seat now held by Rep. Mike Ferguson. The winner faces Assemblywoman Linda Stender in November.
Three Republicans seek the 3rd District seat held by the retiring Rep. Jim Saxton. Democratic state Sen. John Adler awaits the winner.
Also of note is the Democratic contest in the 1st District, where Andrews's wife, Camille, is running to replace him. She faces three Democrats for the nomination.
Andrews has said he will give up his House seat even if he loses to Lautenberg, but others have questioned whether his wife is merely a placeholder while the Democrats decide who to run in the fall. The 1st District includes parts of Burlington, Camden and Gloucester counties.
You can find real-time election returns from all the contested races Tuesday night on 6abc.com
