William "Mo" Cowan poses with his family following a news conference at the Statehouse in Boston, Wednesday, Jan. 30, 2013, where he was named interim U.S. Senator for the seat vacated with the resignation of U.S. Sen. John Kerry, D-Mass. who will become secretary of state. With Cowan are his wife Stacy, and sons Grant, left, and Miles. (AP Photo/Charles Krupa)

Newly-confirmed Secretary of State John Kerry left a vacant Senate seat in Massachusetts and the state’s Governor Deval Patrick announced Kerry’s replacement yesterday. Patrick appointed his former chief of staff, William “Mo” Cowan, to fill the Senate seat for the next five months until a special election is held on June 25th.

Cowan’s appointment means that for the first time in history, there are two African-Americans serving simultaneously in the U.S. Senate. Cowan is also the first black senator to represent Massachusetts in 35 years. While that is a sign of progress, just how much progress is unclear.

Having two black senators this year is a fact that is certainly surprising, since no major Senate candidates in 2012 were black.  The other black senator who will serve alongside Cowan is Tim Scott (R-SC), who was recently tapped by South Carolina’s Republican governor Nikki Haley to succeed retiring Tea Party senator Jim DeMint (R-SC).

There is one common trend that stands out when considering African-Americans and the U.S. Senate: Both of the current black senators are the result of appointments, not elections.

So while the election of the first black president in 2008 is now followed by two black senators serving in the upper chamber, the question remains: Why is it so difficult for the Senate to become a more diverse legislative body, reflecting a more accurate picture of the American electorate?

In fact, there were only six black senators ever in the history of the U.S. Senate before this year: Senators Hiram Revels (R-MS) and Blanche Bruce (R-MS), who were elected immediately after the Civil War, and three more black senators who were elected after Reconstruction, Edward Brooke (R-MA), Carol Moseley Braun (D-IL), and Barack Obama (D-IL).  When President Obama was elected in 2008, his replacement was an African-American, Roland Burris (D-IL), who served until 2010.

It’s not that black candidates haven’t tried to win U.S. Senate races. High-profile candidates including Kendrick Meek (D-FL) and Harold Ford Jr. (D-TN) have tried and failed to mount successful Senate campaigns in recent years.

Meek lost in a tough three-way race in 2010, splitting the vote with former Florida governor Charlie Crist; that left Senator Marco Rubio (R-FL).

Ford fought tooth and nail in a racially-charged campaign in 2006 that left Senator Bob Corker (R-TN).

Other losing black candidates for Senate who lost in the 2010 midterms include the mercurial Alvin Greene in South Carolina and Mike Thurmond in Georgia.

Of course, not all of these black candidates were viable, with any realistic chance of winning.

The end result is these appointed Senators, particularly those who only serve for a short time before a special election ends their tenure, are rarely effective office holders, with little to no power as junior members with no path to leadership. Cowan’s presence in the majority-white chamber would unlikely result in more progressive legislation to assist communities of color and his influence on any important legislation would be minimal.

As a senator, Roland Burris did cast an important vote to get Obamacare passed but it’s unclear what Cowan would be able to vote in favor of in just five months’ time.

The partisan divide in Congress is widening every day. With hotly-contested debates over gun violence prevention, immigration, the debt ceiling and automatic sequestration cuts coming up before the summer, it’s not likely that any of these major issues will be resolved by this summer, based on the pace of Congress’ productivity.

Cowan’s appointment is historic but may simply be symbolic with such a limited capacity for influence.

The next great hope for the Senate comes in the form of Newark mayor Cory Booker, who announced his intention to run for the New Jersey Senate seat currently held by aging senator Frank Lautenberg (D-NJ).

Booker’s success in a statewide race, even in a blue state like New Jersey, could break the mold in terms of the black senator appointed but not elected trend.

Booker appears to be a strong candidate for 2014, but tensions between him and Senator Lautenberg who hasn’t officially announced his retirement is currently making headlines and putting the rising Democratic star’s almost certain Senate success in doubt.

And while Booker’s superhero antics and social media prowess make him an attractive candidate to the mainstream media, only time will tell if he can catapult himself into the U.S. Senate in a little under two years, in what will certainly be his most challenging election yet.

Follow Zerlina Maxwell at @ZerlinaMaxwell