plow the hard ground? what does it mean?

It's not a stock phrase that i know of, but it is understandable. It means undertake something difficult and exhausting. In your context it means that they've done a lot of hard work to get support from both main political parties, never an easy task. The figure comes from the days when farmers would plow their land with a horse. "Hard ground," such as ground which is of poor quality and which has baked in the sun, would exhaust both the man and his horse.

this phrase comes from the bible.
"Break up your unplowed ground
and do not sow among thorns.
(Jeremiah 4:3-4a)

ありがとう，谢谢，thanks！

Closest thing I've heard to that is "a rough row to hoe."

I've only ever heard "a tough row to hoe", Roger, but there's no reason that there couldn't be variations on that theme.

If you study English, you will find that that thing is called a plough.

You assume he/she is trying to learn British?

Hosea 10:12
12 I said, ‘Plant the good seeds of righteousness, and you will harvest a crop of my love. Plow up the hard ground of your hearts, for now is the time to seek the LORD, that he may come and shower righteousness upon you.’
Holy Bible, New Living Translation, (Wheaton, IL: Tyndale House Publishers, Inc.) 1996.

We had to go slow
When we had the old plow
But we're much faster nough
We've got a new plough.