Ballot boxes in Arvada on June 28, 2016.

(Megan Verlee/CPR News)

The contest between incumbent Democrat Michael Bennet and challenger Darryl Glenn, who secured the Republican nomination Tuesday, will be one of the most closely watched U.S. Senate races in the country.

Bennet is widely considered the only incumbent Democrat who may be vulnerable. But Glenn faces an uphill battle in trying to unseat him.

The Washington Post wrote Wednesday morning that Glenn is "the one candidate Senate Republican operatives really, really didn't want to win," given he's a "hard-line conservative" who says he won't moderate his views for a general election.

And Jennifer Duffy of the Cook Political Report has the race "leaning Democrat." Duffy spoke with Colorado Matters host Ryan Warner about that rating.

Editor's note. Jennifer Duffy referred in the interview to Sen. Bennet's "opposition" to the Keystone Pipeline. In fact, Bennet's spokeswoman said, he supports the pipeline and voted for it.

Duffy on Glenn's strengths as a candidate:

"He is something rare in the Republican party, which is an African-American senate nominee... There is a single African-American Republican that sits in the Senate... He's certainly won the hearts of party conservatives."

On whether Glenn's nomination makes it harder for Republicans to win back the  Senate seat:

"I am downgrading the rating of this race this week as a result of Glenn being the nominee... I am saying it's going to be a less-competitive general election... I don't see where Glenn grows his level of support... [And] Senator Bennet is very well funded. Right before the primary they had to file reports with the Federal Election Commission. Senator Bennet had about $7.2 million dollars in the bank and Glenn had about $50,000 in the bank."

On whether national Republican donors will contribute to Glenn's campaign:

"He doesn't have a business background or he doesn't have a big network of donors. As far as outside money goes, the only ads in this race for him were paid for by... a group called the Senate Conservatives Fund, which backs Tea Party candidates, and by the way does not have a great track record in general elections. My understanding of their situation is they don't have the millions of dollars it would take to give Glenn some sort of parity on the air. I don't think you're going to see a lot more outside money come into this race... I don't think you'll see the Chamber of Commerce, for example. I'm not even convinced you'll see money from the national Republican party."

On Bennet's vulnerabilities:

"He barely won this race in 2010 against a very flawed Republican nominee, but it was also a very, very good year for Republicans... But the tables are turned this time. This is going to be a much better year for Democrats overall, partly because of the Republican nominee, Donald Trump, and partly just because of what presidential turnout looks like...

"There are a couple of things in his record that Republicans could mine... His sentiment on the Iran deal is not entirely popular with the electorate. The other is his belief that Guantanamo Bay should be closed and what should happen with those prisoners there. Bennet seems to want the best of both worlds: Please close Guantanamo but please don't send them to Colorado. That's a hard position to navigate."

On what's a stake in this election:

"Control of the Senate is in fact in play. Democrats need to gain four seats if they win the White House, they need five if they don't. There are 10 Democratic-held seats up this time versus 24 Republican-held seats... Republicans are going to be focused almost entirely on keeping their own seats."