Republicans are increasingly optimistic about the party’s chances of taking control of the Senate in November. But even if they achieve that goal, it will be difficult to maintain control of the chamber in 2016.

This year, the Cook Political Report puts just two Republican Senate seats in the “toss up” category — Sen. Mitch McConnell, Ky., and Saxby Chambliss, R-Ga. The rest of the Republican senators seeking re-election this year are considered safe.

In contrast, three Democratic seats are considered either likely to be overtaken by Republicans or leaning that way; five Democrats are in “toss up” seats; and another three Democratic seats are seen as possibly vulnerable. That puts somewhere in the range of eight to 11 Democratic seats potentially in play. Republicans currently have 45 Senate seats, but with Vice President Joe Biden representing the tie-breaking vote, the party would need to gain a net of six seats this year to assume the majority.

But as long as President Obama is still in office, Republicans won't be able to make significant policy gains, regardless of whether they control the Senate. The next best chance to advance a conservative governing agenda is after the 2016 elections, which carries the potential for a new Republican president.

The problem facing Republicans is that while this year Democrats are forced to defend Senate seats that were last won during Obama’s 2008 landslide, in 2016, the reverse will be true. Republicans will be forced to defend Senate seats they won in 2010, one of the best years for the GOP in the party's history.

The vulnerable Republican seats include Democratic-leaning states of Illinois, Pennsylvania, Wisconsin and typical swing states of New Hampshire, Ohio and Florida (where it's unclear how Sen. Marco Rubio's presidential ambitions might interact with the Senate race). Furthermore, Sens. John McCain and Chuck Grassley will both be octogenarians by the 2016 election, and may choose to retire, creating wide-open races in Arizona and Iowa. McCain could face a tough re-election if he does choose to run again. On the Democratic side, the only seats that look potentially vulnerable are the ones in Colorado and Nevada - if Republicans could find a viable candidate to challenge Senate Majority Leader Harry Reid.

What’s more, given that the electorate in midterm elections tends to be more conservative, Republicans who won tough races in 2010 will now be forced to defend them under presidential turnout conditions.

This isn’t to suggest that 2014 doesn’t matter. Quite the contrary — Republicans need to make as many gains as possible while they have Democrats on the defensive. A strong enough year could provide Republicans with a buffer to absorb potential losses in 2016. Republicans are still haunted by the losses of winnable Senate seats in in Delaware and Nevada in 2010 and Missouri and Indiana in 2012.

As Sean Trende of Real Clear Politics has detailed, if Republicans have a lackluster year in 2014, it's possible that Democrats could not only win the presidency in 2016, but also achieve a filibuster-proof Senate majority.

Since Republicans took over the House of Representatives in 2011, they have been largely successfully in blocking Obama from achieving his major policy goals. If a Republican takes over the White House in January 2017, one can only assume that Democrats will return the favor by obstructing his or her legislative agenda as much as possible.

That’s why Republicans will need strong candidate selection and a compelling governing message over the next two elections to make conservative reform ideas a reality.