Bernie Sanders. Joe Raedle/Getty Images Sen. Bernie Sanders is planning on taking his presidential bid all the way to the Democratic National Convention in Philadelphia this summer.

And it looks like his rhetorical shots at Democratic presidential rival Hillary Clinton will go with him.

Leading up to Indiana's primary on Tuesday, Sanders has proved more than willing to continue drawing contrasts with Clinton on major issues.

During a Monday campaign rally in Indiana, Sanders reiterated his usual attacks on Clinton's campaign-finance structure, pausing for boos and cracking jokes about Clinton's private speeches to Goldman Sachs.

"We said, 'Hell no' to super PACs. We don't represent Wall Street or the billionaire class," Sanders said.

He added: "Secretary Clinton has chosen another approach to raise her money. She has not one but several super PACs. In the last reporting period, her major super PAC reported raising $25 million from special interests, including $15 million from Wall Street."

For a moment late last month, it appeared that Sanders might dial back his attacks against his rival, as Clinton's lead in the Democratic race expanded with big victories in primaries in New York and up and down the East Coast.

Speaking to The New York Times last week, senior campaign strategist Tad Devine said that the Sanders campaign would "reassess" its strategy after a string of recent losses. Some political observers noted that the campaign made no mention of Clinton in a statement following Sanders' losses, indicating that he might have been preparing to shift from his rhetorical focus on the frontrunner.

But people close to the campaign have maintained that Sanders is serious about his pledge to participate in a "contested convention." He also reiterated the vow during a Sunday press conference.

Meanwhile, Sanders has continued to cast doubt on the legitimacy of the Democratic presidential-selection process.

On Monday, Sanders used Clinton's massive lead among Democratic superdelegates — elected officials and party officials who represent a small portion of the delegates needed to clinch the nomination — to illustrate how a "rigged system" was built to stymie insurgent candidacies. He again reminded superdelegates that he polls better in hypothetical general-election matchups with Republican frontrunner Donald Trump.

Speaking to supporters on Monday, Sanders reflected on the opposition that his campaign faced in the past year from many Democrats.

"We were taking on the entire Democratic Party establishment," Sanders said, as the audience cheered. "Looks like you're not intimidated by the establishment."

The Clinton campaign has not been pleased by Sanders' persistent attacks on his Democratic presidential rival, which some allies fear could help burnish Republican attacks on Clinton in the general election should she capture the nomination.

On Monday, the Sanders campaign and the Republican National Committee blasted out to reporters a Politico story within four minutes of each other. The story said that the Clinton campaign has benefited massively from the money it has raised for Democratic state parties, which have received comparatively little in return.

"Secretary Clinton has exploited the rules in ways that let her high-dollar donors like Alice Walton of Walmart fame and the actor George Clooney and his super-rich Hollywood friends skirt legal limits on campaign contributions," Sanders campaign manager Jeff Weaver said.

He added an apparent reference to a line from the Politico story that quoted state-party fundraisers as worrying that they were "essentially acting as money laundering conduits" for Clinton and the Democratic National Committee.

"If Secretary Clinton can't raise the funds needed to run in a competitive primary without resorting to laundering, how will she compete against Donald Trump in a general election?" he said.

Donald Trump at the California GOP convention in Burlingame, California. Thomson Reuters

And for his part, Trump has begun incorporating Sanders into his stump speech, repeatedly asserting that he'll attempt to build on Sanders' characterizations of Clinton.

"Bernie Sanders has a message that's interesting," Trump told "Morning Joe" last month. "I'm going to be taking a lot of the things that Bernie said and using them."

He continued: "I can reread some of his speeches and I can get some very good material."

On Sunday, Sanders brushed off Trump's comments.

"The Republican Party and Trump have the resources to do all the opposition research they want on Secretary Clinton. They don't need Bernie Sanders' critiques of the secretary," Sanders said.

But he may not have a choice over how Trump uses his words.

Trump frequently cites Sanders' assertion that "something is clearly lacking" in Clinton's judgment.

"Now, he's saying bad things about Hillary. And he's really correct. He says she doesn't have the judgment to be president," Trump said at a campaign rally in New York last month, referring to Sanders.

Hillary Clinton. Joe Raedle/Getty Images

The Vermont senator is publicly attempting to walk a fine line, maintaining attacks on Clinton while acknowledging that the former secretary of state would be a better choice than Trump.

During an event at the National Press Club on Sunday, Sanders defended his decision to continue drawing contrasts with Clinton, while also acknowledging Trump's overt attempts to co-opt Sanders rhetoric to woo his supporters.

"Secretary Clinton and I have different points of view on a number of different issues. And I have tried my hardest to run an issue-oriented campaign explaining to the American people the differences that we have," Sanders said.

But he quickly added:

Trump is trying in a number of ways to tap into a number of my support. If I lose the nomination, he will not get that support. If I lose the nomination, and we're here to do everything we can to win it, I will fight as hard as I can to make certain that Donald Trump does not become president of the United States.

Clinton has not so subtly attempted several times to transition to the general election over the last several months by praising Sanders and extending an olive branch to his supporters.

"I applaud Sen. Sanders and his millions of supporters for wanting to get unaccountable money out of our politics and giving greater emphasis to closing the gap of inequality," Clinton said during her primary-night victory speech last week.

"And I know together we will get that done," she added. "Because whether you support Sen. Sanders or support me, there's much more that unites us than divides us."