A Yes on May 31 is vital for Taoiseach Enda Kenny if he is to keep his coalition Government between the ditches. A No could be the beginning of the end.
It's all even more crucial for Labour leader Eamon Gilmore at the coalface on this one as Minister for Foreign Affairs.
Gilmore was brilliant in opposition, and always right about where government was wrong.
But he finds being in charge far tougher and much more complex. The frequently-invisible Labour leader badly needs a strong campaign profile -- but above all Gilmore very badly needs a win.
A No vote in summer 2008 on the EU Lisbon Treaty dealt a hammer blow to new Taoiseach Brian Cowen and began a chain of events which set the Fianna Fáil/Green Party coalition on the road to ruin.
Kenny and Gilmore know defeat on May 31 will compound all their ongoing problems and could ultimately prove not just awkward but fatal.
There are other reputations at stake. Labour's Joan Burton is party director of elections and has had a tough time so far in Government. She badly needs a Yes and some suspect she could have been set up for a fall.
Agriculture and Marine Minister Simon Coveney is at the tiller as FG campaign director. He is under less pressure, but a Yes would nicely enhance his rising reputation as he approaches his 40th birthday a fortnight after voting.
There is also a great deal of pressure on the two most visible women campaigners. Lucinda Creighton has cut a dash as junior EU Affairs Minister and often eclipses her department boss, Eamon Gilmore. But her high visibility makes her vulnerable at the hustings.
On the No side Sinn Féin's Mary Lou McDonald has gained very good profile to date as the articulate and good-humoured campaign standard bearer.
She is increasingly tipped to take over party leadership from Gerry Adams. So glitches or foul-ups could prove costly for a woman increasingly looking a future senior government figure.
Fianna Fáil leader Micheál Martin is sitting smack between that political rock and hard place on this one. He managed the strange doings of 'Dev Óg', aka Éamon Ó Cuiv, rather well.
But Martin still has Sinn Féin out there on his green flank and a No would further erode the scarce and shaky ground beneath him and FF. Martin needs a Yes every bit as badly as Gilmore does.
Elsewhere on the Yes and No side the pressure comes down markedly by comparison.
Joe Higgins and Richard Boyd Barrett's United Left Alliance will consolidate their positions and have potential to pick up votes from people who always voted for their bigger parties, but are now struggling to find work and keep their homes. They just have to keep doing what they're doing.
Other No campaigners such as Donegal South West independent Thomas Pringle, who is taking a court challenge to the whole thing, and his unlikely ally, Shane Ross, can gain valuable profile from a platform they are very comfortable being on.
As so often happens, the No side have far less to lose here. A No victory would be superb for their political standing. But a heightened turnout and a respectable No vote can -- with some good grounds -- be claimed as bankable political credibility.
Across the Yes side, defeat could start bringing the curtain down on careers, with Labour leader Eamon Gilmore and Fianna Fáil leader Micheál Martin looking most vulnerable.
Winning is about turnout (see panel). And good turnout is only driven by the parties mobilising their own local supporters.
John Downing was a government press adviser 2007-2011, and also reported from Brussels for the Irish Independent from 1989-1999.