While many pundits and political observers were quick to praise Hillary Clinton's pick of Tim Kaine as her running mate on Friday, the choice wasn't met with universal acclaim.

One important group — progressives and backers of Clinton's former rival, Vermont Sen. Bernie Sanders — were not as pleased with the selection of the Virginia senator who has cultivated a reputation of working across the aisle over the course of his political career.

Progressive Change Campaign Committee co-founder Stephanie Taylor said in a statement that with some of Kaine's pro-free trade positions, "Republicans now have a new opening to attack Democrats on this economic populist issue" that Republican nominee Donald Trump has also been vocally opposed to.

That's a weakness Clinton's campaign seemed to recognize, and a Clinton campaign aide told NPR's Tamara Keith that the senator had reconsidered his position on supporting fast-tracking for the Trans-Pacific Partnership deal and couldn't support it in its current form.

But even before Kaine was announced on Friday, other groups remained skeptical of his appeal. Democracy for America executive director Charles Chamberlain called Kaine's support for bank deregulation "disqualifying."

Many such groups would have preferred more progressive champions such as Massachusetts Sen. Elizabeth Warren or Ohio Sen. Sherrod Brown. Both were reportedly on Clinton's short list, but their home-state GOP governors would have appointed a Republican to replace them if Clinton wins, further tipping a Senate chamber Democrats are trying to flip.

Here are four reasons why progressive groups are unhappy with Kaine as the new vice presidential nominee.

1. He's too nice

In an election year where the Democratic base fell in love with the pugnacious styles of Sanders and Warren, Kaine's low-key, nice-guy persona feels like a bit of a letdown. Clinton is picking a Democrat whom Republicans like, which in this polarized climate feels like a betrayal. He's the type of lawmaker who seeks to build bipartisan consensus, not the kind who holds the line on the liberal position. Arizona Republican Sen. Jeff Flake — who has refused to endorse Trump — even had good words to say about Kaine after the news broke.

2. He's too white

A Southern, white Democrat is not exactly the future of the party. Not picking a woman or a minority also feels like a disappointment among base voters. Especially if they feel like women (Warren) and Latinos (Labor Secretary Tom Perez) were passed over for the white guy.

3. He's too moderate

Except, Kaine isn't really. He has a zero percent rating from the American Conservative Union. But his bipartisan style often lends itself to a perception that he's a moderate. On some issues, he definitely strikes a more centrist tone. Kaine is big on the military, partly because that's a huge issue in Virgina, home to many military bases and defense contractors. And he's also seen as a business-friendly Democrat, which is true because he was a governor and most governors have more business-friendly views on how to govern.

Kaine has supported some tweaks to Dodd-Frank Wall Street reforms that liberals see as a betrayal. He's also a devout Catholic who is personally opposed to the death penalty and abortion, but he has a 100 percent rating from Planned Parenthood.

4. He's too "safe"

Some (including Kaine himself) say he is boring. He won't overshadow the top of the ticket but will be an effective on-message surrogate. Clinton has admitted she isn't a strong retail politician, while Kaine has managed to win statewide in Virginia twice. He hasn't been tested yet on a national stage and may not be the firebrand attraction some had hoped for. But he could help her attract disaffected Republicans and independents turned off by Trump.

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