Sen. Tim Kaine of Virginia, the Democratic vice presidential nominee, and Rep. Elijah E. Cummings (D-Md.) greet diners at L.P. Steamers restaurant in Baltimore after speaking at the National Urban League Conference last week. (Jabin Botsford/The Washington Post)

Sen. Tim Kaine of Virginia is holding fast to his long-held belief that the current military operation against Islamic State forces has not been properly approved by Congress — a position that puts him at odds not only with President Obama but also with his running mate, Democratic presidential nominee Hillary Clinton.

Kaine placed the blame squarely on Congress for its inability to tackle the issue.

“I don’t think the current legal authorities are sufficient to wage this war against ISIL,” Kaine said on NBC’s “Meet the Press” Sunday, using an acronym for the Islamic State.

As Obama’s first secretary of state, Clinton supported the president’s position that he had all the authority he needed to wage military action in Syria, Iraq and other trouble spots without first seeking congressional approval.

While she has supported Kaine’s push for more congressional action, Clinton has also been clear that she will continue to act under the same authority Obama has, even when Capitol Hill doesn’t weigh in.

Sen. Tim Kaine (D-Va.) is Democratic presidential candidate Hillary Clinton's vice presidential candidate. Here's what you need to know about him. (Sarah Parnass,Osman Malik/The Washington Post)

Kaine’s ongoing, unsuccessful effort to draft and win approval for a new war resolution serves as a window into how Kaine views the relationship between the executive and legislative branches. It has also shown the limitations of his ability to coax allies across the finish line, even on his hallmark issue — suggesting that his courtship approach might not work in today’s hyper-partisan era.

Finally, how Kaine handles this issue going forward will offer telling evidence of his influence within the next administration, should the Democrats win.

The first clue came when he delivered his 30-minute acceptance speech for the vice-presidential nomination in Philadelphia last month — without mentioning the war powers issue.

Nowhere did Kaine describe how much he has tried to compel Congress to define the parameters of an increasingly hot war.

After winning election in 2012, Kaine led the effort to draw up a new war resolution to replace those written more than a decade ago to take on Osama bin Laden and then Saddam Hussein. Almost single-handedly, he turned what congressional leaders in both parties had considered a nuisance into a defining cause about congressional duty and the constitutional boundaries a president faces.

“The unwillingness of this Congress to authorize the war not only shows a lack of resolve, it sets a dangerous precedent,” Kaine said in early June, introducing amendments to try to force a debate on the issue. “It’s not hard to imagine a future president using this inaction to justify the hasty and unpredictable initiation of military action.”

But Kaine, 58, and a small band of younger allies in the Senate failed. They never even got full consideration in the Senate Foreign Relations Committee, on which he serves.

Even Kaine’s close friends recognize that he now must adhere to whatever posture Clinton takes on this issue.

“Tim knows he’s applying for a job that is very different,” said Sen. Chris Murphy (D-Conn.), a close ally who sits next to Kaine on the Foreign Relations Committee.

However, Kaine’s generation of lawmakers hope that in choosing her running mate, Clinton demonstrated that she wants to engage Congress on a new authorization for the use of military force, as war resolutions are technically known.

“Hillary is signaling that she’s serious about an AUMF,” Murphy said.

Given the past two years of gridlock on the issue, Kaine faces long odds of being any more successful on crafting and passing a war resolution as Clinton’s vice president.

It had been an area of concern for several years, but in the summer of 2014 — as the Islamic State took over more land in Syria and Iraq while committing high-profile murders of Western hostages — Obama ramped up a bombing campaign and sent more U.S. troops into those hostile territories to help advise Iraqi military leaders.

At issue is the president’s authority to take the country to war without first seeking congressional approval. Obama has claimed that congressional authorizations passed in the wake of the terrorist attacks of Sept. 11, 2001, permit him to order any actions combating terrorism. And he has bluntly said that he has no timeline for how long the forces would be deployed.

Kaine has said that the post-Sept. 11 authorizations were too “open-ended,” arguing that actions in Iraq, Afghanistan, Libya and Syria all required separate congressional approval. One of those resolutions was meant to target bin Laden, who was killed in 2011; the other dealt with Hussein, who was captured, tried and killed by the new Iraqi government in 2006.

In an early 2015 interview, after Republicans took the Senate, Kaine said that he would judge Majority Leader Mitch McConnell’s stewardship of the chamber on a single issue, whether he allowed a full war debate on the Senate floor.

One of the ironies in the fact that it never happened is that Republicans were fully willing to approve a broad, sweeping authorization giving Obama, and any future president, almost unlimited power to go after the Islamic State. It was the Democrats, including Kaine, who wanted to place limits on both time and deployment of ground troops on any future authorizations.

“There’s no reason for us to give him less authority than what he has today, which is what he’s asking for,” John A. Boehner (R-Ohio), the House speaker at the time, told reporters after a 2015 trip the Middle East.

Key Republicans, including Senate Foreign Relations Chairman Bob Corker (R-Tenn.), agreed with the president’s view that the 2001 war declaration gave the administration all the authority it needed, according to a GOP official reviewing the debate this year.

Kaine can sound like a tough military hawk calling for a war debate, but his own position is squarely on the liberal side of the argument. He wants a strict timeline — one proposal called for a three-year war against Islamic State forces, after which the next president would have to withdraw or get new authority to act. He wanted limits on ground forces in those territories, which most Republicans consider a non-starter.

Murphy called Kaine a “strong defender” of Obama’s use of forces so far. “His bone of contention has been that the strategy has been unauthorized,” Murphy said.

Kaine also could never win over his own leadership. Sen. Harry Reid (D-Nev.), the majority leader in 2014 and now minority leader, essentially told Kaine to drop the matter, Sen. Christopher A. Coons (D-Del.) and other Democrats said.

“This is an issue that’s easy to brush aside,” Coons said.

Reid preferred letting Obama act on his own, according to a Democratic official familiar with the 2014 and 2015 discussions. Irritated by Reid’s inaction, Kaine used direct channels into the West Wing, emailing senior advisers to try to get them to push Reid, the Democratic official said.

But Kaine could never persuade Obama to push Congress to act. Obama periodically raised the issue and said Congress should pass a new resolution, but he never gave it top priority and was content continuing to make his war decisions with no congressional input.

Ultimately Kaine even lost the support of his longtime allies on the issue. Both Murphy and Coons said that as the Republican presidential primary unfolded, they lost interest in the war resolution bid. The Republican presidential nominee, Donald Trump, and the second-place finisher, Sen. Ted Cruz (R-Tex., both advocated a forceful use of the military.

Democrats simply decided they couldn’t risk compromising with Republicans and giving more power than they were comfortable with, if Trump ended up winning.

“It’s a different animal today,” Murphy said.