Former Secretary of State Hillary Clinton is a candidate for the Democratic nomination for president. The opinions expressed in this commentary are hers.

America had just elected a Democratic president. The Senate was majority-Democratic. The House of Representatives had more Democrats than at any time since 1992.

The Republicans could either change themselves -- by embracing the young, diverse and tolerant America of the 21st century -- or they could try to change whose voices count in our democracy.

We all know the cynical path they chose. Six years ago Thursday, the Supreme Court's decision in Citizens United transformed our politics by allowing corporations to spend unlimited amounts of money to influence elections.

The effect was immediate. In the 2010 midterms, outside groups spent nearly 60% more than in 2006. In 2014, the top 100 donors spent nearly as much as all 4.75 million small donors combined. National networks of big-money donors funneled mountains of cash into targeted state races. By 2014, one party controlled both the governor's mansion and the state legislature in 36 states -- the most since the 1950s.

Restricting voting rights

Meanwhile, Republicans launched an aggressive campaign to restrict voting rights across the country. After the 2010 election, lawmakers in 41 states introduced at least 180 measures designed to make it harder for people to vote. This ugly effort got a boost in 2013 when the Supreme Court made another disastrous ruling, striking down hard-won Voting Rights Act protections. Today, 21 states have new laws restricting voting rights.

And some of the greatest damage to voting rights has come in states with single-party control -- like North Carolina, which eliminated same-day voter registration and slashed early voting; Florida, which instituted such heavy restrictions on voter registration drives that the League of Women Voters had to shut down operations; and Alabama, which passed a strict photo ID requirement and then closed dozens of driver's license offices across the state, making it much harder for citizens to get the IDs they now need to vote.

Put it all together -- the flood of corporate money in our elections, the rise of single-party control of state governments, the sharp increase in voting-rights restrictions -- and the result is unmistakable. Our democracy is being hollowed out. And that should offend every American, no matter what party you belong to.

Reclaim our democracy

We can't let this continue. It's time to reclaim our democracy, reform our distorted campaign finance system and restore access to the ballot box in all 50 states.

That starts with reversing Citizens United. And that's where my comprehensive plan to restore common sense to campaign finance begins. As president, I'll appoint Supreme Court justices who recognize that Citizens United is bad for America. And if necessary, I'll fight for a constitutional amendment that overturns it.

Meanwhile, we need more transparency in our politics. In the last three elections, more than $600 million in donations came from unknown, untraceable sources. That's a lot of secret, unaccountable money. As president, I'll require federal contractors to fully disclose their political spending. I'll call on the Securities and Exchange Commission to require that publicly traded companies do the same. And I'll fight for legislation requiring the disclosure of all significant political donations, no matter where they come from or who they benefit. Whether you're a Democrat or a Republican, you should have to identify your donors.

We should also make it easier for Americans to run for elected office. You shouldn't have to be rich or well-connected to serve. I'll fight to create a robust small-donor matching system, so people with good ideas and a passion for public service know that they can run without having to court big donors and special interests.

Make it easier to vote

Finally, we have to do a much better job of protecting Americans' voting rights. Nothing is more vital to our democracy. I'll fight to restore the full protections of the Voting Rights Act . And I'll go further, because we should be making it easier to vote, not harder.

All Americans should be automatically registered to vote on their 18th birthdays, unless they opt out. Every state should have at least 20 days of early in-person voting. And no one should ever have to wait more than 30 minutes to cast a ballot.

Citizens United and its aftermath have twisted and perverted our democratic system. Now the deck is stacked even more in favor of those at the top. But as Al Smith, another Democrat from New York, once said, "All the ills of democracy can be cured by more democracy." So that's what we need now: more transparency, more accountability, and above all, more citizens exercising their right to vote.

This November, we can show Republicans that they made the wrong choice eight years ago. Let's restore people's voices and people's votes to their rightful place -- at the center of our democracy.