It’s no secret that Rep. Bob Menendez (D-N.J.) has his eye on the Senate. And lately, the Democratic Caucus chairman seems to be trying the chamber on for size.

The debate over use of the “nuclear” option was perhaps the most glaring example of the New Jersey Democrat working to burnish his Senate credentials. Menendez weighed in with press releases and even managed to be included as the lone House Member among leading Senators in a photo opportunity after last week’s compromise was struck.

“He seemed right at home over in the Senate Russell Office Building,” quipped one senior House Democratic aide of a Roll Call cover shot last week that featured Menendez alongside Minority Leader Harry Reid (Nev.) and Sen. Byron Dorgan (N.D.). “He’s certainly trying to appear more Senatorial.”

“He’s spending time doing what’s required” of a Senate candidate, added a Democratic Member, speaking on the condition of anonymity. “He’s doing what he has to do.”

Menendez has been running a relatively quiet but consistent campaign for a vacancy that doesn’t yet exist in the Garden State. He’s been working to increase his profile and banking more than $2.5 million for a prospective Senate run that would follow a hoped-for appointment to the chamber late this year or early next.

Menendez, like several of his colleagues including Reps. Frank Pallone and Robert Andrews, is hoping that Sen. Jon Corzine (D), if elected governor in November, will tap him to serve as that state’s junior Senator. If he gains Corzine’s appointment, Menendez would then have to run for the seat in November 2006.

“Menendez is in a tough spot,” said one well-placed Democratic aide. “Anybody who runs for statewide office — [Reps.] Harold Ford [Tenn.], Ben Cardin [Md.], Ted Strickland [Ohio] — spends every morning, noon and night running for it.

“He wants so badly to be in the United States Senate,” the staffer added. “And it doesn’t have two years to do it, it’s a six-month sprint.”

Because of that, sources said Menendez is trying to make inroads early — trying to start Senate relationships and show he has the makings of a Senator. He has participated in at least three Senate-side press events in as many weeks, and has begun to weigh in on more statewide New Jersey issues — including opposing efforts to privatize Amtrak and the possible closure of Fort Monmouth.

One Senate Democratic leadership aide said because of his House leadership slot, role as a prominent Hispanic and overall ability to articulate the Democratic message, Menendez has been asked to work with Senate leadership on top issues and participate in recent events. In no way is Menendez muscling his way into Senate affairs, the aide insisted.

“He’s been very helpful lately,” said the staffer. “I think any help we get from the House is always good, but there are issues on which he is very good and has some very important things to say. That’s why we invite him.”

Menendez spokesman Matt Miller said his boss is simply “working hard to do his job fighting for New Jersey families.”

And, Miller added: “That often involves working with the Senate on issues important both nationally and in New Jersey. His leadership in Washington benefits New Jersey and would certainly also be helpful in any future opportunities.”

But the more time he spends across the Capitol, the better his chances could be if and when Corzine makes a decision about whom to appoint as a successor, Democratic Senate and House sources said.

Corzine is likely to turn to Senate leadership — including Reid and Democratic Senatorial Campaign Committee Chairman Charles Schumer (N.Y.) — for counsel. By forging relationships with Reid, Schumer and others, Menendez may have a leg up on other prospective replacements, sources said.

A Menendez adviser said part of the New Jersey lawmaker’s strategy includes increasing his interaction in the Senate, but more importantly Menendez is trying to be ever more active on issues that affect the entire state and introduce himself to a statewide audience.

This aide said Menendez wants to show New Jersey voters that he “will fight for them” and that he “would be as strong a fighter as a Member of the Senate as he has been in the House.”

“He’s certainly stepped his activity all around,” noted one Democratic source with ties to New Jersey.

This source added that while Menendez may be trying to appear more Senatorial, he also appears to be trying to “make up for lost time,” having not paid a lot of attention to some of these statewide issues in the past.

“All of a sudden he’s interested” in these issues, the source said.

Ford, who just announced his bid for the Democratic Senate nomination in Tennessee, said Menendez is playing it smart and that it only makes sense to get to know the chamber and potential future colleagues.

Ford noted that he has already struck solid friendships with Southern Sens. Mary Landrieu (La.), Mark Pryor (Ark.) and Blanche Lincoln (Ark.), and he will spend part of the Memorial Day recess in Iraq on a Congressional delegation trip with Sen. Joseph Biden (Del.).

“It’s not a prerequisite, but it can’t hurt” to forge early relationships, Ford said. “There’s no doubt about it.”

Cardin, who is also running for Senate, said while he couldn’t speak directly to Menendez’s efforts, he feels comfortable with his personal Senate ties given his near two decades of House service. He said he expects his transition to the other chamber would be seamless, and said, “I think it would probably be the same for Bob Menendez also.”

Unlike some of his colleagues with Senate ambitions, Menendez must balance his New Jersey political ambitions with his job as Caucus chairman, the No. 3 leadership post. Sources throughout the Caucus said Members are taking notice that Menendez is spending less time in the House lately.

“His mind is certainly elsewhere,” said the Democratic Member.

The Menendez adviser said being a Democratic House leader gives Menendez the opportunity to be more active and participate in more events on the Senate side. The aide did acknowledge, however, “there is a balancing act” for the Democratic lawmaker, but “he continues to fulfill his responsibilities in House leadership.”

Several sources said it’s unlikely Menendez will feel any backlash as a result of his divided attentions given he isn’t the House leader, and that he continues to handle the key aspects of his Caucus chairman job.

“The average Member doesn’t view it as that big a deal; he’s not the Whip, he’s not the leader,” said one House Democratic aide. “It has not risen to the level of angst and concern of [then-Minority Leader Richard] Gephardt [D-Mo.], who was on his presidential ambitions when he was the leader of the House Democrats.”